Nanocrystalline tungsten carbide (WC) synthesis/characterization and its possible application as a PEM fuel cell catalyst support

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Electrochimica Acta, 61, pp. 198-206, 2011-12-09

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Nanocrystalline tungsten carbide (WC) synthesis/characterization and

its possible application as a PEM fuel cell catalyst support

Zhu, Weimin; Ignaszak, Anna; Song, Chaojie; Baker, Ryan; Hui, Rob; Zhang,

Jiujun; Nan, Feihong; Botton, Gianluigi; Ye, Siyu; Campbell, Stephen

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ContentslistsavailableatSciVerseScienceDirect

Electrochimica

Acta

jo u r n al h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a

Nanocrystalline

tungsten

carbide

(WC)

synthesis/characterization

and

its

possible

application

as

a

PEM

fuel

cell

catalyst

a_Institute_for_Fuel_Cell_Innovation,_National_Research_Council_of_Canada,₄₂₅₀_Wesbrook_Mall,_Vancouver,_BC,_V6T_1W5_Canada

b_Department_of_Materials_Science_and_Engineering,_McMaster_University,₁₂₈₀_Main_Street_West,_Hamilton,_ON,_L8S_4L8_Canada

c_Ballard_Power_Systems_Inc.,₉₀₀₀_Glenlyon_Parkway,_Burnaby,_BC,_V5J_5J8_Canada

d_AFCC_Automotive_Fuel_Cell_Cooperation,₉₀₀₀_Glenlyon_Parkway,_Burnaby,_BC,_V5J_5J8_Canada

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received5October2011

Receivedinrevisedform1December2011

Accepted2December2011

Available online 9 December 2011 Keywords:

Tungstencarbide

Catalystsupport

Oxygenreductionreaction

PEMfuelcells

a

b

s

t

r

a

c

t

Nanocrystallinetungstencarbide(WC)withahighsurfaceareaandcontainingminimalfreecarbonwas

synthesizedviaapolymerroute.Itsphysicalproperties,includingsolubilityinacidsolution,electronic

conductivity,andthermalstability,werethoroughlystudiedattwoelevatedtemperatures:95◦_C_and

200◦_C._Compared_to_commercially_available_WC,_this_in-house_synthesized_WC_showed_lower_solubility

inacidicmediaat200◦_C,_higher_electronic_conductivity_(comparable_to_that_of_carbon_black),_as_well_as

higherthermalstability.However,thismaterialexhibitedlowelectrochemicalstabilityinacidicmedia

whensubjectedtopotentialcyclingatpotentialslargerthan0.7Vvs.RHE,duetotheelectrooxidation

ofWC.ThemajorproductofWCelectrooxidationisWO3,whichwasconfirmedbyX-rayphoton

spec-troscopymeasurements.PtwasuniformlydepositedonthehighsurfaceareaWCtoforma20wt%ofPt

supportedcatalystfortheoxygenreductionreaction(ORR).TheORRmassactivitywasthenobtained

usingtherotatingdiskelectrodetechnique.

1. Introduction

Inthelastseveraldecades,protonexchangemembrane(PEM) fuel cells, which have several advantages such as high power density,highenergydensity,andlow/zeroemission,have demon-stratedtheirfeasibilityaspracticalenergyconversiondevicesfor manypower-demandingfieldsincludingportable,automotive,and stationary applications. One of the major technical challenges, insufficientdurability,is identifiedasa barrier hindering wide-spreadcommercializationofPEMfuelcells.Ithasbeenrecognized thatthedegradationoffuelcellcatalystsisthemajorcausefor insufficientdurability.

Atthecurrentstateoftechnology,carbon-supportedplatinum (Pt)-basedcatalystsarethemostpracticalforPEMfuelcell opera-tionintermsofbothcatalystactivityanddurability.Unfortunately, duringfuelcelloperation,Ptdissolutionandcarbonsupport oxida-tion/corrosionwilloccur,degradingthecatalystperformance,and leadingtolowdurabilityofthefuelcell.Toaddressthischallenge, oneapproachistoreducecarbonsupportcorrosionbyhybridizing

∗ Correspondingauthor.Tel.:+16042213000x5577;fax:+16042213001.

E-mailaddress:chaojie.song@nrc.gc.ca(C.Song).

thesupportmaterialsorreplacingsomeofthecarboncontentusing non-carbonmaterials.Withrespecttothis,intensiveresearchhas beendonewitha focusonthecorrosion mechanism ofcarbon supportanditspreventioninthemostrecentyears[1–3].

Currently, carbon blacks,suchas Vulcan XC-72Rand Ketjen black,havebeenusedasthecatalystsupportsforPEMfuelcell catalystsbecauseof theirunique propertiesincludingsufficient electronic conductivity, high surface area, and suitable porous structure.However,underfuelcelloperatingconditions,carbon ispronetooxidationthroughthefollowingreaction:

C+H₂O→CO₂+4H++4e−, Eo= 0.207Vvs.SHE (1) At the fuel cell cathode, the normal operation potential (0.5–0.9V vs. RHE)is more positive than the carbon oxidation potential.Duringthestart-up/shutdownprocess,thepotentialcan reachas high as1.5V vs. RHE (reversible hydrogenelectrode), significantlyhigherthanthecarbonoxidationpotential.These elec-trodepotentialsfacilitatecarbonoxidation throughreaction(1). Furthermore,thepresenceofPtand/orhightemperature opera-tionofthePEMfuelcellcanacceleratecarbonoxidation/corrosion. Obviously,thiscarbon oxidation/corrosioncanresultinthe iso-lation of Pt catalyst particles and Pt catalyst agglomeration, degradingthefuelcellperformancesignificantly[4].Therefore,the

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developmentofhighperformancenon-carbonsupportmaterial,to replacecarbonisimperative.This,however,turnsouttobenot aneasytask,becauseahighperformancecatalystsupportshould meetallofthefollowingrequirements:highthermalstability,high electronicconductivity,lowsolubilityinacidicmedia,high electro-chemicalstability,highsurfacearea,aswellasfavouriteinteraction withthecatalystparticles.

Amongthenon-carbonsupportcandidates,tungstencarbide (WC)appearstobeattractive.Asarefractorymetalcarbide tra-ditionallyforerosionresistantcoatings,WC hasa highthermal stabilityandischemicallyinert.Theexperimentalresultsshowed that, under ambient conditions, the oxidation of WC starts at around600◦_C _[5],_much_higher_than_the_operating_temperature ofPEMfuelcells.WCalsoexhibitsgoodelectronicconductivity, whichmakesitanexcellentmaterialfordiffusionbarrierlayers inthesemi-conductorindustry.Inaddition,thePtlikecatalytic behaviourofWCcouldresultinasynergeticeffectwiththecatalyst, leadingtoanincreaseincatalyticactivity[6].

Tungsten carbide supported Pt catalysts have been synthe-sizedand usedfortheoxygenreduction reaction(ORR)aswell asmethanoloxidation, and havedemonstrated theirsynergetic catalytic effect. Meng and Shen [7] prepared a carbon-based nanocrystalline W2C using the intermittentmicrowave heating (IMH)method,thenmixed withPt/CandIMHtreated againto formacatalyst.Theirresultsshowedthatthecompositecatalyst obtainedcouldexhibitahigherORRonsetpotential,higher spe-cificactivity,aswellasbetteroxygenreductionselectivityinthe presenceofmethanolthanthatofthePt/Ccatalyst, demonstrat-ingasynergeticeffectbetweenthetungstencarbideandPt.Nie etal.[8]synthesizedaPt–WC–W2C/Ccatalystusinganimproved one-stepIMH.ThePt–WC–W2C/Ccatalyst showedaneven bet-terORRactivitythanthosePt–W2C/Ccompositecatalysts.Wang etal.[9]preparedhighsurfaceareatungstencarbidemicrospheres (withmixedWCandW2Cphases)usingahydrothermalpolymer method.ThePt/tungstencarbidemicro-spherescatalystwith uni-formlydistributedPtparticlesexhibitedabetterORRactivitythan thePt/Ccatalyst,whichwasattributedtothelarge electrochemi-calsurfaceareaandthePt–tungstencarbidesynergeticeffect.Zhu etal.[10]observedanenhancedORRcatalyticactivityforPton tungstencarbide-modified-carbonsupport.Modificationofcarbon withtungstencarbidecouldalsoimprovethethermalstabilityof thesupport.GanesanandHam[11–13]synthesizedhighsurface areatungstencarbide(W2C)microspheresandWCsupportsusing acarbonspheretemplate.ThecatalystwithPt(7.5%)loadingon theW2C microsphereshowedhigheractivitytowardsmethanol electro-oxidationthantheconventional20%Pt–Ru/Vulcancatalyst [11–13].Thisispossiblyduetoaco-catalyticeffectbetweenPtand tungstencarbide:tungstencarbideactivatesmethanoltoforma methoxyintermediate,andPtpromotesthedecompositionofthe methoxyspecies[12].

Thehighthermalstability,highelectronicconductivity,aswell asthesynergeticcatalyticeffectwiththecatalyst,maketungsten carbidepromisingasaPEMfuelcellcatalystsupportespeciallyfor hightemperatureoperation.However,theelectrochemical stabil-ityofWCmightbeanissuethatcouldpreventWCfromapplications inPEMfuelcells. Controversialliteratureresultswerereported regardingtheelectrochemicalstabilityofWCandWCsupported catalysts.MazzaandTrassatti[14]usedWCasaninertelectrode, whichdemonstratedgoodchemicalandelectrochemicalstability belowtheoxygenevolutionpotential.Chhina[15]foundthatWC supportedPtcatalystwaselectrochemicallymorestablethanthe carbonsupportedcommercialcatalystHiSpec4000TM_(Pt/Vulcan XC-72R)whensubjectedtocontinuousCVscansatelevated tem-perature(80◦_C)._Voorhies_[16]_found_that_WC_could_be_chemically oxidizedin1MH2SO4atroomtemperature,andelectrochemically oxidizedatmediumtohighanodicpotential.Weigertetal.[17]

observeda largeanodicpeak at0.6–0.8Vonthecyclic voltam-mograms(CVs)ofWCthinfilm,whichwasattributedtotheWC oxidation,andWO3 wasindeeddetectedbyXPSonthesurface aftertheCVscans.Lee[18]alsofoundthatWCwasnot electro-chemicallystable.However,somemetalsadditionscouldprevent orslowdowntheelectrochemicaloxidationofWC,possiblydueto asynergeticeffect.Forexample,theelectrochemicaloxidationof WCcouldbesloweddownbyalloyingwithTa,andtheWCthinfilm, onwhichamono-layerofPtwasdeposited,remainedunchanged afterCVmeasurements,whiletheonewithoutPtdepositionwas completelyoxidizedintoWO3.

Theimprovedelectrochemicalperformanceofthetungsten car-bidebasedPtcatalyst[7–13],andthecontroversialresults[14–18] regardingtheelectrochemicalstabilityoftungstencarbide moti-vatedtheauthorstomakeathoroughinvestigationofthephysical, chemical,andelectrochemicalpropertiesofWCtoevaluateits pos-sibilityasaPEMfuelcellcatalystsupport.

Itisworthwhiletonotethatintheliterature,aconsiderable amountof workwasfocused ontungstencarbide/carbon com-positesupports.Sincecarbonwillbeelectrochemicallyoxidized, leadingtofuelcellperformancedegradation,theexistenceof car-boninthesupportmaynotbedesirable.Inaddition,thetungsten carbidereportedinsomepapersisamixedphase(W2CandWC), whichisnot suitableforPEMfuelcellcatalystsupport applica-tionbecauseW2Ciselectrochemicallyandthermodynamicallyless stablethanWC[19].Fromthepointviewoffundamental under-standing,acarbon-free,single-phaseWCsupportisdesiredandis thefocusofthispaper.

Inourhightemperature(HT)PEMfuelcellapproach,inorderto overcomethechallengeofcarbonsupportcorrosioninHTPEMfuel cells,wesynthesizedasingle-phaseWCsupportwithhighsurface area,andinvestigateditselectronicconductivity,chemicalstability inacidicsolution,aswellastheelectrochemicalstability.ForORR activity,thisWCmaterialwasusedasthePtcatalystsupportto formPt/WCcatalyst,andthecorrespondingORRmassactivityas wellastheelectrochemicalstabilitywereinvestigated.

2. Experimental

2.1. SynthesisofWCsupportandPt/WCcatalyst

Tungstencarbide(WC)supportsweresynthesizedusinga poly-merprecursorroutemodifiedbasedonGanesan’sprocedure[7,8]. Typically, 4.73gof ammoniummetatungstate(AMT, Fluka) and 1.21g of resorcinol (Sigma–Aldrich) were dissolved in 1.63mL of formaldehyde (37% in H2O, Sigma–Aldrich) and 19.8mL of deionizedwater.Thesolutionwasthenrefluxedat95◦_C_for₂₄_h forminganorange precipitate.Theprecipitate waswashed and driedatroomtemperature,andthenheat-treatedinatubefurnace underamixtureofAr(flowrate:160mL/min)andH2(flowrate: 60mL/min)gasflowat1000◦_C_for₂_h_to_form_WC._Afterwards,_the furnacewascooleddownto700◦_C_and_kept_for₂_h_under_a_H

2flow (flowrate:115mL/min)toremovetheextracarbon.The synthe-sizedWCisdenotedasWC-SYNinthispaper.ForPtdepositionto formWC-supportedcatalyst(Pt/WC),anintermittentmicrowave heating(IMH)assistedpolyolmethodwasemployedtodepositPt ontotheWCsupport.Typically,90mgWCsupportwasdispersed in20mLethyleneglycol(EG),andthepHwasadjustedto8.0with 0.2MNaOHEGsolution.ThisWCsuspensionwasthensonicated for1h,followedbyadding46.6mgofH2PtCl6 (dissolvedin3mL EG)dropwise intotheWC suspension.Afterwards,thecatalyst precursorsuspensionwasheatedinamicrowaveoven(LBP111RS, LaddResearch,USA)at600Wfor60s,andthenintermittentlyfor 6timeswith10sofmicrowavingeachtime.Theproductwasthen filtered,washed,anddried.

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2.2. Structureandmorphologycharacterizations

ThestructureoftheseWCsampleswascharacterizedbyX-ray diffraction(XRD)andthelatticepatternoftheWCsupportwas observedusinghighresolutiontransmissionelectronmicroscopy (HRTEM)togetherwithelectrondiffraction.Theinter-planar dis-tance of the lattice was measuredbased on the latticefringe. Scanning electron microscopy (SEM) and transmission electron microscopy(TEM)wereemployedtostudythemorphologiesof theWCprecursors,WCproducts,andPt/WCcatalysts.

TheXRD measurementswere carried outusing a Bruker D-8diffractometerwithastepsizeof0.02◦ _and _an_exposure_time of 0.1s for each step. SEM observations were carried out on a Hitachi S3500N scanning electron microscope with an oper-ating voltage of 20kV. Samples for TEM characterization were directlysupportedona coppermeshwithacarbon micro-grid. TEMinvestigationsatrelativelowmagnificationwereperformed using a Philips CM12 operated at 120kV. HRTEMobservations were carried out using a JEOL-2010F electron microscope at an accelerating voltage of 200kV with a Field-Emission Gun (FEG). This instrument is equipped with a Gatan Imaging Fil-ter (GIF) for energy-filtered imaging and an energy dispersive X-ray(EDX)detectorforelementalanalysisandmapping. Exper-imentallyobtainedelectron diffractionpatternswerecompared to electron diffraction patterns simulated with the software programJEMS.

2.3. Solubilitytest

Thesolubilityofcommerciallyavailable(AlfaAesar)and synthe-sizedWCpowdersweretestedin1MH2SO4 at95◦Cand200◦C, respectively,accordingtothefollowingprocedures:forthe solubil-itytestat95◦_C,_a_flask_containing₂₀₀_mg_of_WC_powder_and₅₀_mL of1MH2SO4wasputinanoilbathwithconstantstirring,keptat thistemperaturefor24h.Afterwards,thesuspensionwasfiltered, andthefilteredsolutionwassentforICP/MS(inductivelycoupled plasmamassspectroscopy)toanalyzeforW,fromwhichthe sol-ubilityofWC wascalculated.Theleftoverpowderwaswashed, dried,andsubjectedtoXRDcharacterizationtolookforanyphase changes;forthesolubilitytestat200◦_C,_an_in-house_made chemi-callyinertvesselwithapressuresealingsystemwasused.200mg ofWCpowdersampleand50mLof1MH2SO4wereputintothis vessel,whichwasthenfilledwithN2gasatapressureof200psi.The highpressure(200psi)keptinsidethevesselcanpreventH2Ofrom evaporatingat200◦_C,_so_that_the_{concentration}_of_H

2SO4doesnot changeduringthetest.Thevesselwasthenheatedinanoilbath at200◦_C_for₂_h._After_cooling_and_{depressurization,}_the suspen-sionwasfiltered,andthesolutionanalyzedusingICP/MSwhere theleftoverpowderwasthenwashed,dried,andsubjectedtoXRD characterization.

2.4. Thermalstabilitytest

Thethermal stabilitytest wascarried outat200◦_C _for₉₆_h. Typically,certainamountofsamplewasputinaMuffleFurnace, heldat200◦_C_in_air_for₉₆_h._Then_the_sample_was_cooled_down toroomtemperatureandsentforXRDcharacterizationtoobserve anyphasechanges.

2.5. Conductivitymeasurement

Anin-housemadeconductivitycell,whichconsistsofanUltem cylinder(innerdiameter=0.5cm)toholdthesampleandtwoCu pistonsaselectrodes,wasusedforconductivitymeasurement.A smallamountofWCpowderwasplacedintheconductivitycell, andaloadwasappliedontheupperpistontokeepapressureof

80kPa.ThewholesetupwasputinaMufflefurnacetocontrolthe temperature.ATsuruga3566ACm-Ohmtesterwithafour-point setupwasusedtomeasuretheconductivityataconstantfrequency of1kHz.TheelectronicconductivityofWCwasmeasuredat95and 200◦_C.

2.6. Otherphysicaltests

Forsurfaceareaanalysis,aBeckmanCoulterSA-3100Surface AreaAnalyzerwasemployedtoobtainN2adsorption/desorption isotherm,fromwhichtheBETsurfaceareaoftheWCsamplewas calculated.Forthedeterminationoffreecarboncontentinthe syn-thesizedWCsamples,thermalgravityanalysis(TGA)usingaPerkin ElmerParis6ThermalGravityAnalyzerwascarriedoutontheWC samplesfromroomtemperatureupto730◦_C_in_air._The_X-ray pho-toelectronspectroscopy(XPS)oftheas-synthesizedWCpowder andtheoneafter700cyclicvoltammogramscans(from0.05Vto 1.2V,seebelow),takenbyaLeyboldMAX-200X-rayPhotoelectron Spectrometer,wasusedtoanalyzethesurfacecompositionsofthe samples.

2.7. Electrodepreparationandelectrochemicalmeasurements Toprepareacatalyst(Pt/WC)orsupport(WC)ink,19mg cat-alystorsupportwasdispersedin9.5mLisopropanoland0.5mL de-ionizedwater,sonicatedfor1htoformacatalystorsupport ink.24␮Lofcatalystorsupportinkwasuniformlydepositedona glassycarbonelectrodewithageometricareaof0.20cm2_(Glassy carbon electrode diameter: 5mm, AFE7M050GC, Pine Research Instrument)toformacatalystorsupportlayer.Afterwards,7.1␮L Nafion®_solution_(50:1_in_methanol)_was_deposited_on_top_to_form theelectrodecoatinglayer.

Theelectrochemicalmeasurements werecarriedout usinga conventionalthree-electrodecellcontaining0.1MHClO4aqueous solution.ForORRmeasurement,thiselectrolytesolutionwas sat-uratedwithpureoxygen(bubbledbypureoxygengas).For CV, thissolutionwasbubbledwithpurenitrogengastoremoveall dis-solvedoxygen.AnRHE(reversiblehydrogenelectrode)wasusedas thereferenceelectrodeandaPtwireasthecounterelectrode.The measurementsweretakenonaSolartron1480multi-potentiostat, controlledbyCorrwaresoftware.Fortheelectrochemicalstability test, the electrode wasimmersed in 0.1M deoxygenated (bub-blingwithN2)HClO4 solutionat30◦C, andtheCVcurves were recorded from 0.05V to 1.2V with a scan rate of 20mV/s up to1000scans.From theCV curves,theelectrochemicalsurface areawascalculatedfromhydrogenadsorption/desorptionpeaks between0.05Vand0.4V.FortheORRmeasurements,the voltam-mogramwasrecordedwithaRDE(rotatingdiscelectrode)ina 0.1MO2-saturatedHClO4 solutionwitha scanspeedof 5mV/s. AnASRrotator(PineInstrument)wasusedtocontroltherotation rate.

Forelectrochemicalactivesurfacearea(ECA)measurement,CO strippingmethodwasused.Thepotentialofthecatalystloaded electrodewillbeheldat0.05Vvs.RHEin0.1MHClO4 withCO purginguntilsaturationofCOadsorptionisreached.Theelectrolyte solutionispurgedwithN2for30mintoremoveun-adsorbedCO. Thenthepotentialoftheelectrodewillbescannedfrom0to1.0V vs.RHEatascanrateof20mV/s.TheECAwillbecalculatedfrom thechargeoftheCOoxidationpeakaccordingtoEq.(1′_):

ECA = QCOadsorption

0.42 ×1000 (1

′₎ whereQCOadsorptionisthechargeforCOoxidationinC,and0.42mC isthechargerequiredtooxidizeamonolayerofadsorbedCOon 1cm2_of_Pt_surface.

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Table1

PropertiesofWCsamples.

BETsurfacearea(m2_/g) _Solubility_(wt%) _Conductivity_(S/cm) _Thermal_stability

95◦ C 200◦ C 95◦ C 200◦ C WC-Alfa 1.6 0.15 0.63 0.75 0.87 Stableupto200◦_C WC-SYN 89 0.16 0.27 2.4 3.0 Stableupto200◦_C

3. Resultsanddiscussion

3.1. PropertiesofcommerciallyavailableWC

As a preliminary assessment of WC as a catalyst support, thesolubility,electronicconductivity,thermalstability,and elec-trochemicalstability of commerciallyavailable WC (denotedas WC-Alfahereafter)wereinvestigated.Thesurfaceareaofthis cat-alystisonly1.6m2_/g.

Asshown inTable1,thesolubilityofWC-Alfain1MH2SO4 at95and200◦_C_is_0.15_wt%_and_0.63_wt%,_{respectively.}_No_phase changewasobservedontheXRDpatternsaftersolubilitytests(not shownhere),indicatingthatthismaterialhasalowsolubilityand highchemicalstabilityin1MH2SO4.TheWC-Alfawasalsofound tobethermallystableat200◦_C._No_phase_change_on_the_XRD pat-ternwasobserved(notshownhere)after96hofheattreatmentat 200◦_C_in_air._The_electronic_conductivity_of_the_WC-Alfa_was_found toincreasewithtemperaturefrom0.75S/cmatroomtemperature to0.87S/cmat200◦_C,_which_is_in_the_same_order_as_that_of_carbon black.

ThecyclicvoltammetryscanswereconductedonWC-Alfato evaluateitselectrochemicalstability.Nooxidation/reductionpeaks wereobservedwithinthepotentialrange(0.05V–1.2V)upto1000 CVscans(notshownhere).Thisindicatesthegoodelectrochemical stabilityofWC-Alfa.

BasedontheabovepreliminaryassessmentresultsonWC-Alfa, itseemsthatWCcouldbeasuitablecandidateasaPEMfuelcell catalystsupport.However,thesurfaceareaofthisWC-Alfaisfairly low(only1.6m2_/g)._In_order_to_obtain_high_ORR_performance,_a highsurfaceareaWCisdesirable.Wehavethussynthesizedahigh puritynano-crystallineWCwithhighsurfacearea.

3.2. Synthesisandpurificationofnano-crystallineWC

Fig.1showstheXRDpatternofthesynthesizedtungsten car-bide(WC-SYN).Itcanbeseenthat apureWC phasewithonly

90 80 70 60 50 40 30 47 48 49 50 2θ (Degree) Inte nsity (a.u. ) WC (2 0 1 ) WC (1 0 2 ) WC (2 0 0 ) WC (1 1 1 ) WC (0 0 2 ) WC (1 1 0 ) W(110) WC (1 0 1 ) WC (1 0 0 ) WC (0 0 1 ) Intensity (a.u.) 2θ (Degree) WC (101)

Fig.1.XRDpatternofin-housesynthesizedWCsample(WC-SYN).

smallamountofWmetal(0.9wt%basedontheXRDpeakarea) wasobtained, indicating thatWC wassuccessfully synthesized. Usually,WCsynthesis involveshightemperature,which results inlowsurfaceareaproducts.However,apolymersynthesis pro-cedurecanreducethesynthesistemperaturesignificantly.Inthis polymersynthesisprocedure,aBakelitetypeW–Ccomplex precur-sorwasformedasanintermediatephase[20].Thisintermediate W–Ccomplexphase couldresult ina W–Chomogeneityatthe molecularlevel,leadingtosignificantdecreaseoftheW–Calloying temperaturecomparedtothatforsolid-statesynthesis.

The WC phase obtained in this work is different from that reported in the literature. For example, Ganesen et al. [11,12] showedthatthetungstencarbideproductwasdominatedbyW2C. Normally,inWCsynthesis,thereduction/carburizationundergoes thefollowingsequence:WO3–WO2–W–W2C–WC[21,22]. Forma-tionoftungstencarbiderequiresmetallicWandactivecarbonthat isproducedontheWsurface.Thephaseofthetungstencarbide productcanbedeterminedbytwocriticalsteps:theformationof activecarbonC*ontheWsurface,andthediffusionofC*intothe metallicW[22].IfthereductionoftungstenoxideintoWisslow, theamountofC*formedwillbelowduetothelackoffreshW surface,andthediffusionofCintoWwillbefast,leadingtothe formationof W2C.On theotherhand,ifthereductionof tung-stenoxideintoWisfast,thereisenoughWfortheformationof C*,and thediffusionofCintoWwillbecomethelimitingstep. Asaresult, WCphase willbeformed.Inthis work,inorderto obtainpureWCphase,weincreasedtheWprecursor(ammonium metatungstate)contenttocreatemoreWavailableforthe forma-tionofC*,andchangedthegasenvironmentfrompureArtomixed gasflowofArandH2tospeedupthereductionoftungstenoxide intoW.Thesechanges successfullyshowtheformationof pure WCphase.

Fig.2showstheTGAresult(inair)ofthesynthesizedWC-SYN after2hofH2treatment.Aweightgainstartingfrom400◦Ccanbe observed,whichisattributedtotheoxidationofWCintoWO3.This weightgainreachesitsmaximumataround600◦_C,_and_then_starts todecrease,whichisduetoburningofexcesscarbon.After650◦_C,

700 600 500 400 300 200 100 0 0 5 10 15 Weigth Gain (%) T (oC)

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theweightgainremainsconstant,indicatingthattheproducthas beencompletelyoxidized.Itisworthwhiletonotethatinthis WC-SYNsample,somefreecarbonexists,whichisnotdesirable.

Inordertoremovecarbon(especiallyonthesurface),themost commonwayisflushingthesamplewithH2atanelevated tem-peratureviaamethanizationreaction.However,H2canalsoreduce WCintoWdependingontemperature,heattreatmenttime,aswell asH2concentration.Sincethemethanizationreactionhasalower GibbsfreeenergythantheWCreductionreaction,itmightbe pos-sible,byoptimizingtheseexperimentalparameters,toremovethe excesscarbonatareasonableratewithoutsignificantformationof metallicW.Ribeiroetal.[23]reportedthatWCsurfaceabsorbed polymericcarbon,whichcouldberemovedafterheattreatmentin H2at700◦Cfor1.5h.Howeverafter2hofheattreatment, metal-lictungstencouldbedetectedbyXRD.Theoptimalconditionof carbonremovalbyH2treatmentdependsontheparticlesizeand surfacepropertiesoftheWCproductaswell.

Inthiswork,wetreatedtheas-synthesizedWCproductat700◦_C for2hinaH2atmosphere.AsshowninFig.2,thecompositionof theWCproductcanbefoundtobe94.2wt%ofWC,0.9wt%ofWand 4.9wt%ofC,assumingthatWC,W,andCaretheonlycomponents. WealsoobservedthatextendingH2treatmenttimedidnotlead tofurtherreductioninCcontent,ratheritincreasedWcontent. However,whiledecreasingH2treatmenttime,ahigherCcontent wasobserved.Therefore,2hofH2treatmentat700◦Cseemedtobe theoptimaltimetoremovethesurfacecarbonontheWCproduct. Itisbelievedthattheremaining4.9wt%ofcarbonisinsidetheWC particles.

3.3. Propertiesofthein-housesynthesizedWC-SYN

Thesurfacearea,solubility,electronicconductivity,aswellas thethermalstabilityofWC-SYNweretestedinthesamewayas thoseforcommerciallyavailableWC-Alfa.Table1summarizesthe results.Forcomparison,thepropertiesofWC-Alfawerealso pre-sented.

ThesurfaceareaofWC-SYNwasmeasuredusingN2 adsorp-tion/desorption isothermand theobtained datawas compared withthatofWC-Alfa.Fromtheisothermcurve(Fig.3),aBETsurface areaof89m2_/g_was_obtained_for_WC-SYN,_which_is_{significantly} higherthanWC-Alfathatisonly1.6m2_/g.

ThesolubilityofWC-SYNat200◦_C_is_0.27_wt%,_lower_than_that_of WC-Alfa.Aftersolubilitytesting,astrongWO3peakcouldbeseen ontherecoveredWC-SYN,whileontherecoveredWC-Alfasample, therewasnosuchWO3peak.Thisobservationmaysuggestthatthe formationofinsolubleWO3ontheWC-SYNshouldberesponsible foritslowersolubility.

TheconductivityofWC-SYNmeasuredat95and200◦_C_is_listed inTable1togetherwiththoseforWC-Alfaforcomparison.Itcan beseenthattheWC-SYNhasahigherconductivitythanWC-Alfa.

1.0 0.8 0.6 0.4 0.2 0.0 0.1 1 10 WC-SYN WC-Alfa Adsorbed Volume (cm 3 /g) P/P₀

Fig.3.N2adsorption/desorptionisothermcurvesforWC-AlfaandWC-SYN.

ThishigherconductivityofWC-SYNmightbepartiallyduetothe existenceofWmetal,causedbypostH2treatment.

Regardingthethermalstability ofWC-SYN,noimpurephase couldbeidentifiedontheXRDpatternafterthesamplewasheated inairat200◦_C_for₉₆_h,_indicating_high_thermal_stability_of_WC-SYN. 3.4. WC-SYNmorphologiesandstructure

Fig. 4 shows the SEM images of the WC-SYN precursor (a) andproductaftercarbonremoval(b).TheWC-SYNprecursorhas asphericalmorphologywithsizesaround0.5–1.0␮m.However, afterthesyntheticprocessandheattreatment,thesemicrospheres collapsedasseeninFig.4bandFig.5.Thisresultisdifferentfrom theliterature[11],wheremicrospheresmorphologywasretained inthefinalproduct.Thiscollapseofthemicrosphereinthefinal WCproductmightbecausedbylowercarboncontentinthe pre-cursor.Normally,carboncanactasskeletonforthemicrosphere.If thecarboncontentintheprecursorisbelowacriticalextent,the microspherestructuremightcollapse.Thebeneficialaspectisthat thiscollapsecanexposemoresurfacesforcarbonremoval,leading toalowCcontent.

Fig.5shows theTEMimageofWC-SYN.Nano-particleswith morphologyofrice-likenanocrystalsandwhiskerscanbeclearly observed.Theaverageparticlesizeisabout35nmaccordingtothe measurementover200particles(Fig.6).

In orderto furtherinvestigatethestructure ofWC-SYN, the HRTEMimagingtogetherwiththeelectrondiffractionwascarried out.AsshowninFig.7,theWC-SYNexhibitsaclearlatticefringes withwell-definedsinglecrystal-likeelectrondiffractionpattern (Fig.7c), indicating high crystallinity of WC-SYN particles. The

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Fig.5. TEMimageofWC-SYNparticles.

primaryinterplanarspacingsona lowindexzone axisare2.57 and2.92 ˚A,respectively(Fig.7c),whichareattributedtothe(010) and(001)reflectionsandconsistentwiththehexagonalWClattice (spacegroupofP-6m2).

3.5. Pt/WC-SYN

Tomakeacatalystfortheoxygenreduction reaction,Ptwas depositedontheWC-SYNsupportbyapolyolmethod.Fig.8shows theTEMimageof20wt%Pt/WCcatalyst.Itcanbeseenthatthe PtdistributionontheWCsupportisfairlyuniform,demonstrating thesuccessinsynthesizingPt/WCcatalyst.TheaveragePtparticle sizewasfoundtobearound5nm.

3.6. ElectrochemicalstabilityofWC-SYNandPt/WC-SYN

InordertotesttheelectrochemicalstabilityofWC-SYNsupport anditssupportedPtcatalyst(Pt/WC-SYN),thesematerialswere coatedontheelectrodeindividually,thentestedinN2-saturated 0.1MHClO4solutionat30◦Cusingcyclicvoltammetry.Fig.9shows thecyclicvoltammograms(CVs)ofWC-SYN.Inthefirstcycle,a largeanodiccurrentwasobservedatpotentials>0.7Vvs.RHE.This

Fig.6. SizedistributionofWC-SYNparticles.

Fig.7.(a)EnlargedHRTEMimageofWC-SYNparticle;(b)lowermagnificationviewofaparticle;and(c)thecorrespondingindexedselectedareaelectrondiffraction(SAED)

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Fig.8. TEMimageof20wt%Pt/WC-SYNcatalyst(averagePtparticlesize:5nm).

peakdecreasesastheCVscanproceeds,andfinallydisappearsat the100thCVscan.Atthesametime,redoxpeaksbetween0.1and 0.4Vvs.RHEonbothanodicandcathodicscansbecomemoreand moreprominentwithincreasingCVscan,andthenslightlydecrease after100CVcycles,especiallyforthecathodicpeaks(Fig.9b).

Thelargeanodiccurrentat>0.7V vs.RHEshouldarisefrom theelectrochemicaloxidationofWC,asreportedintheliterature [17–19].WiththeproceedingofCVscan,moreandmoreWCwas oxidizedintoWO3, which coveredtheWC surface, resultingin graduallydiminishingofthiscurrent.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 I (mA/cm 2 ) E (V) vs RHE 1st CV scan 10th CV scan 30th CV scan 100th CV scan CV of WC support

(a)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 200th_CV_scan 600th_CV_scan 1000th_CV_scan CV of WC support I (mA/cm 2 ) E (V) vs RHE

(b)

Fig.9.CyclicvoltammogramsofWC-SYNinaN2-saturated0.1MHClO4solution.

(a)First100scans,and(b)remaining900scan.Scanrate:20mV/s,temperature:

30◦

C,andWCloadingontheelectrode:48␮g/cm2.

42 40 38 36 34 32 30 28 (a) WC as-synthesized

Binding Energy (ev) (b)

WO 3

WC WC after 700 CV scans

Intensity (A.U.)

Fig.10. XPSspectraofWC-SYN(a)beforeCVscanand(b)after700CVscans.

Thepeaksbetween0.1and0.4Vvs.RHEcouldbeattributed tothereversibleredoxprocess ofWO3/HxWO3 inacidic media [24–26]:

WO3+xH++xe−↔HxWO3 (2)

At thecathodicscan, proton intercalatesinto WO3, forming hydrogen tungsten bronze, resulting in cathodic peaks. At the anodicscans,whenthepotentialincreasestoacertainvalue,H+_will de-intercalatefromthehydrogentungstenbronzelattice, oxidiz-ingHxWO3backtoWO3,leadingtoanodicpeaks.Theintensitiesof theseanodicandcathodicpeakswereexpectedtoincreasewithCV cyclingbecausemoreWO3wasproduced.However,adecreaseof thepeakintensitieswasobservedafter100CVscans.Thisislikely duetothedecreaseinsurfaceareacausedbytheWO3coverage [26].

InordertoconfirmtheformationofWO3 ontheWCsurface, XPSspectraweretakenonWC-SYNbeforeandaftertheCVscan, asshowninFig.10.Fortheas-synthesizedWC,aW4fdoubletwith bindingenergiesof31.5eVand33.6eV,canbeobserved,which cor-respondstotheWCphase.Inaddition,twopeakscanalsobefound athigherbindingenergy(35.2eVand37.4eV),correspondingto theW4f7/2andW4f5/2 peaksofWO3phase(Fig.11a).The pres-enceofsmallamountofWO3onthesurfaceoftheas-synthesized

1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 CV of Pt/WC catalyst I (mA/cm 2 ) E (V) vs RHE 1st CV scan 20th CV scan

Fig.11.Cyclicvoltammogramsof20wt%Pt/WC-SYNinN2-saturated0.1MHClO4

solutionat30◦

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1.2 1.0 0.8 0.6 0.4 0.2 -6 -5 -4 -3 -2 -1 0 2500 rpm 1600 rpm 900 rpm 400 rpm 100 rpm I (mA/cm 2 ) E(V) vs RHE

Fig.12.Voltammetriccurvesatdifferentrotationrates,recordedonarotating

diskelectrodecoatedwith20wt%Pt/WC-SYNcatalystinO2-saturated0.1MHClO4

solutionat30◦_C._Scan_rate:₅_mV/s._Pt_loading:_47.8

␮g/cm2.

WC-SYNmightbeduetotheoxidationofWCunderambient con-ditions[18].After700CVscans,theintensityoftheW4f_7/2 and W4f5/2peaksincreasedsignificantly(Fig.11b),indicatingan elec-trochemicaloxidationofWCintoWO3.TheseXPSresultsconfirm theconversionofWCintoWO3onthesurfaceoftheWC-SYNas aresultofCVcycling.Itisinagreementwiththosereportedby Weigertetal.[17]andLeeetal.[18].

CVcyclingwasalsoconductedonthePt/WC-SYNcatalystto test the electrochemical stability of WC supported Pt catalyst. Fig.11showstheCVcurveofthe20wt%ofPtdepositedon WC-SYN(20wt%Pt/WC-SYN). Largehydrogenadsorption/desorption (from0.05Vto0.4V)peaks,anddisproportionallysmallPt oxida-tion/reductionpeakswereobserved.TheshapeoftheCVcurvefor the20wt%Pt/WC-SYNcatalystisverysimilartothatofthe WC-SYNsupport(after100CVcycles),andthepeakpositionsforthe lowpotentialregion(0.1Vand0.3V)matchwellwiththatofH+ intercalation/de-intercalationprocessinWO3.Similarphenomena werealsoreportedintheliterature[27].Therefore,thislarge hydro-genadsorption/desorptioncurrentiscomposedoftwoprocesses:H adsorption/desorptiononPtandhydrogenadsorption/desorption onWO3.

Fig.11alsoshowsthattheintensityofthePtoxidationpeak decreasesas theCV scan proceeds. This is possibly due tothe electrochemical oxidation of WC-SYN. The formation of non-conductiveWO3couldisolatePtparticlesdepositedonit,leadingto electrochemicallyinaccessiblePtparticles.However,thelarge oxi-dationcurrentat>0.7VonbareWC-SYN(Fig.9)wasnotobserved on20wt%Pt/WC-SYNcatalyst,indicatingthattheoxidationofWC inPt/WC-SYNisnotassevereasbareWC. Thismeansthatthe presenceofPtonWCmightstabilizeWCorinhibitWCoxidation, consistentwiththeresultsreportedinliterature[17,18].

3.7. ElectrocatalyticactivityofPt/WC-SYNtowardsoxygen reductionreaction

InordertoevaluatetheelectrocatalyticactivitytowardORR,the in-housesynthesized20wt%Pt/WC-SYNcatalystwasdepositedon theelectrodesurfacetoformacatalystlayer,thenputintoan elec-trochemicalcellforORRmeasurements.Fig.12showstheCVcurves recordedatvariouselectroderotationratesat30◦_C_in_O

2-saturated 0.1MHClO4 solution.AnORRmassactivityof3.3mA/mgPtwas obtainedfromthe1600rpmcurveat0.9Vvs.RHEafterthe lim-itingcurrentcorrection.Thisvalueissignificantlylowerthanthat

-0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 1.2 1 0.8 0.6 0.4 0.2 0 -0.2 E (V) vs RHE I (mA/cm 2 )

Fig.13.TheCOadsstrippingvoltammogramon20wt%Pt/WC-SYNin0.1MHClO4

at30◦

C.Scanrate:20mV/s,Ptloading:48␮g/cm2.

obtainedwithcommercialPt/C(110mA/mgPt[28]).Inthe litera-ture,largerORRmassactivitieswerereportedforPt/WCcatalysts suchas12mA/mgPt[7],and24mA/mgPt[8](convertedfrom spe-cificactivityandPtloading).However,inthesecases,largeamount ofcarbonwaspresentinthecatalysteitherintheformofPt/Cand WCmixture[7]orPtdepositedontheWC+Ccompositesupport [8].Obviously,partoftheORRmassactivitycomesfromthePt depositedonC.Inaddition,thePtloadinginPt/WCisveryhigh (from50%to100%)[8].IfthehighdensityofWCisconsidered, suchahighPtloadingcouldleadtotheformationofaconducting Ptmultilayer.ElectronsinvolvedintheHOR(hydrogenoxidation reaction)andORRdonotneedtogothroughthesupportandthe catalystbehaveslikePtblack,resultinginhighORRperformance.

InordertobetterunderstandwhatcausesthelowORRmass activity,theECAwasmeasuredbyCOstripping,whichissupposed toavoidWCsupportinterference.Fig.13showstheCOstripping voltammogram.AclearCOoxidationpeakbetween0.6–0.8Vwas observed.FromEq.(1′_),_the_ECA_was_calculated_to_be₉_m2_/g_Pt._For Ptwithparticlesizeof5nm,thetheoreticalexternalsurfacearea shouldbe56m2_/g_Pt._This_indicates_that_only_16%_of_the_external surfaceiselectrochemicallyactive,whichmightbethemaincause ofthelowORRmassactivity.TheECAcanalsobecalculatedby subtractingtheHdesorptionpeakonWC(Fig.9)fromthatonPt/WC (Fig.11),whichgivesavalueof45m2_/g._This_value_is_close_to_the theoreticalvalueandsignificantlylargerthanthatobtainedwith COstripping.Mostpossibly,theinteractionbetweenPtandWC enhancedtheHadsorption/desorption.Howeveritdidnotaffect O2adsorptionandreductiononPt,whichisevidencedbythesmall PtoxidationpeakinFig.11.

Tafeldiagramofthelinearscanwasplotted.The>0.9V poten-tialregionwasselectedfortheTafelplot,becauseinthatpotential region,theORRcurrentisindependentoftheRDErotationrate, asobservedinFig.12,indicatingapurekineticbehaviour.Fig.14 shows the Tafel plot of the Pt/WC-SYN catalyst. A linear rela-tionbetweenlogarithmcurrentdensityandpotentialisobserved, whichhasaTafelslopeof77mV/dec,largerthanthe60mV/dec usuallyreportedforPtcatalyzedORRreactionatlowoverpotentials [29].ThislargerTafelslopeindicatesthatthislowmassactivityis partlycausedbythesluggishreactionkinetics.

ThelowPtutilizationandlargeTafelslopeobtainedwiththe Pt/WC-SYNmightbeduetothecontactresistancebetweenthe catalystandthesupportifthesupportisoxidized.Aninsulating WO3layermayblocktheelectrontransferbetweenthecatalystand thesupport,isolatingthePtcatalystresultinginlowPtutilization. Inaddition,therearemanyfactorsthatcancausealowmass activity. For example, the non-uniform distribution and/or the agglomeration ofthePtcatalyst onthesupportcoulddecrease Pt utilization, unfavourable electronic interaction between the

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4.9 4.8 4.7 4.6 4.5 4.4 0.90 0.91 0.92 0.93 Potential (V) log I

Tafel slop: 77 mV/dec

Fig.14. TafelplotoftheORRobtainedwith20%Pt/WC-SYNcatalystinO2-saturated,

0.1MHClO4,30◦C.Scanrateof5mV/s,andRDErotationrate:1600rpm.

catalyst and thesupportcouldalsoshift theonset potentialof ORRtonegativepotential,andsoon.Thesetopicsaredefinitely acontinuingresearchareaforfuturework.

4. Conclusions

Nano-crystallineWCwithasurfaceareaof89m2_/g_and min-imalcarboncontentwassynthesizedusingapolymerroute.The in-housesynthesizedWC(WC-SYN)hasahighelectronic conduc-tivity,highthermalstability,aswellaslowsolubilityinacidwhen comparedtocommerciallyavailableWC-Alfainthetemperature rangeof95◦_C–200◦_C._However,_WC-SYN_has_a_slightly_lower elec-trochemicalstabilityduetotheelectrooxidationintoWO3,which wasconfirmedbyXPSmeasurementsaftertheWCsamplewas potentialcyclinginthepotentialrangeof0.0–1.2Vvs.RHE.This instabilitywasattributedtoitshighsurfaceareabecausethe com-merciallyavailableWCsample,whichhasamuchlowersurface area,wasfoundtobeelectrochemicallystable.

WC-SYNwas testedas a Ptcatalyst support.Uniformly dis-tributedPtparticleswereobservedonWC-SYN.Electrochemical measurementsshowedthatthepresenceofPtdidnotcatalyzeWC oxidation.Incontrary,itcouldslightlystabilizetheWC-SYN sup-port.However,themassactivityofORRona20wt%ofPtsupported onWC-SYNwasonly3.3mA/mgPt,significantlylowerthan com-mercialPtsupportedoncarbon.Moreworkshouldbefocusedon

improvingthemassactivityandthesupportstabilityoftheWC supportedPtcatalysts.

Acknowledgements

Financial supportfrom NationalResearch Council of Canada (NRC),InstituteforFuelCellInnovation(IFCI)NRC,BallardPower Systems, andAFCC (Automotive FuelCell Cooperation)through theTechnologyDevelopmentProgram(TDP)isgreatlyappreciated. TheauthorswouldliketothankMr.KenTsayatNRC-IFCIfortheCO strippingmeasurement.Partoftheelectronmicroscopywas car-riedoutattheCanadianCentreforElectronMicroscopy,afacility supportedbyNSERCandMcMasterUniversity.GABisgratefulfor aNSERCstrategicgrantforpartiallysupportingFNandthiswork. References

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